1. Field of the Invention
The present invention relates to a method for measuring cloud height by measuring the transit time of a light pulse emitted from the ground upwardly into an air space to be investigated and reflected back to the ground by a cloud. The measuring range is divided into a plurality of different height bands which are scanned sequentially starting from the lowest height band and extending up to the highest height band at the top end of the measuring range. A train of incident light pulses is emitted from a ground-based transmitter up into the investigated air space and signals from each one of these height bands, reflected towards a ground-based receiver, are integrated and measured.
In this specification the reference to "light" should be taken to include electromagnetic radiation at suitable frequencies in the infra red and ultraviolet regions of the spectrum and not just visible light.
The determination of cloud height by transit time measurement requires a measuring of the time which elapses from the emission of a light pulse from the transmitter until reflections of an at least pre-defined strength are received by the receiver.
2. Description of Prior Art
One method of cloud height measurement generally similar to that to which this invention relates is described in U.S. Pat. No. 3,741,655. The method described in U.S. Pat. No. 3,741,655 consists of dividing up the measuring range--by means of gate pulses which are time-delayed step-by-step and which control the signal detector in the receiver--into height bands or intervals and filtering a number of echo signals from the respective height interval, whereby the signal-to-noise ratio increases with the root of the number of filtered echo pulses (the number of "samples"). Since the aim is to detect echo signals from a cloud, echo signals from water droplets and other particles in the air space below the cloud, as well as the attentuation caused by these, can be regarded as a disturbance which, for a measuring sequence with a fixed time-controlled sensitivity control and/or a fixed energy input of the measuring pulses, may result in incorrectly measured values of cloud height or even make it impossible to detect clouds which do exist. Operating with a fixed energy input of the measuring pulses, it is necessary to use a receiver and signal detector having a very large dynamic range. The method of cloud height detection previously used employed an emitted light energy which was constant and was thus not related to the level of atmospheric attentuation currently occurring in the atmosphere below the respective measuring band. This known method gives, under many weather situations, an incorrect balancing between the level of incident energy supplied and the demand for measuring energy at different height levels in order to obtain a satisfactory measuring signal. In addition, the previous method makes use of a relative measurement between two relatively adjacently positioned height intervals, which does not provide any possibilities of compensating for a varying light signal attenuation in the atmosphere.